Mass filter with removable auxiliary electrode



MASS FILTER WITH REMOVABLE AUXILIARY ELECTRODE Filed March 17, 1969 Mal ch 17 1970 w. M. BRUBAKER 2 Sheets-Sheet 1 4 IN VENTOR.

Mam M Jew/2m mwmyz March 17, 1970' w. M. BRUBAKER MASS FILTER WITH REMOVABLE AUXILIARY ELECTRODE Filed March 17, 1969 2 Sheets-Sheet z T I: /F max/fame INVENTOR.

United States Patent M 3,501,630 MASS FILTER WITH REMOVABLE AUXILIARY ELECTRODE Wilson M. Brubaker, Arcadia, Califl, assignorto Bell & Howell Company, Chicago, Ill., a corporation of Illinois Continuation-impart of application Ser. No. 568,259,

July 27, 1966. This application Mar. 17, 1969, Ser.

Int. Cl. B01d 59/44; H01j 39/34 U.S. Cl. 250-413 Claims ABSTRACT OF THE DISCLOSURE CROSS REFENCE TO RELATED APPLICATIONS This is a continuation-in-part of application Ser. No. 568,259, filed July 27, 1966, now abandoned.

SUMMARY OF THE INVENTION This invention relates to mass filters and in particular to multipole mass filters utilizing centrally located auxiliary electrodes.

A non-magnetic mass analyzer, such as that described in U.S. Patent 2,950,389, is normally referred to as a quadrupole mass filter. Such an instrument is usually energized by connecting sources of alternating and fixed voltage to the electrodes of the filter in a predetermined relationship to produce an analyzing electric field in the area located internally of the electrodes. When energized in this manner the quadrupole has been found to have a discriminating or analyzing capability with respect to charged particles which are introduced into the field Within the rods. When such particles are introduced into this field they are deflected under the influence of the field and, depending on their mass-to-charge ratio, assume stable or unstable trajectories in the course of their transit through the filter. Those particles with unstable trajectories are normally deflected toward one of the rods and discharged thereon. Particles having stable trajectories follow characteristic paths of motion through the filter and exit therefrom to impinge on some form of charged particle collector.

The operation of such a filter is normally described in terms of a so-called stability diagram, components of motion in the x and y plane, and the location of the working point for particles of a given mass within the stability diagram. In a stability diagram, a parameter, which is proportional to the DC voltage applied to the rods of the filter, is plotted against a second parameter which is proportional to the AC voltage applied to the rods of the filter. Particles introduced into the filter assume trajectories which, for analysis purposes, can be br ken down into x-components of motion and y-components of motion. Particles of certain predetermined mass-tocharge ratios have Working points which fall within the stable portion of the stability diagram and are transmitted by the analyzer. Particles having other than the 3,501,630 Patented Mar. 17, 1970 desired mass-to-charge ratio have working points which fall outside the stable region of the diagram just previously referred to and these particles are the ones deflected to and discharged upon the rods of the filter.

Another analytic instrument similar to the quadrupole mass filter is a monopole mass filter such as that described in U.S. Patent 3,197,633. This filter resembles a quadrupole in that it is also a non-magnetic instrument which utilizes the combined etfects of static and alternating electric fields created in a certain region to separate charged particles according to their mass-to-charge ratio. The monopole, as its name indicates, utilizes a single field electrode in contrast with the four field electrodes of the quadrupole. A primary advantage of the monopole in comparison to the quadrupole is the simplicity of its structure and supporting equipment. Additional advantages of the monopole relative to the quadrupole are recited in U.S. Patent 3,197,633.

The present invention provides an improved quadrupole structure whose mode of operation can be converted to that of the monopole and dualpole thereby combining several types of analyzer instruments in a common basic physical configuration. The invention is a multipole mass filter for selectively detecting charged particles and comprises a plurality of substantially parallel rod electrodes symmetrically disposed about and.

spaced from a central axis and from each other to define a central interelectrode space located generally along the axis and four remote interelectrode spaces located between each pair of adjacent rod electrodes. A source of AC voltage, a source of DC voltage, and a source of reference voltage are provided, together with means for applying opposite phases of AC voltage to a first and second pair of opposed rod electrodes to create an alternating multipole electric field within the filter. Means for applying the positive side of the DC source to the first pair of opposed rods and means for applying the negative side of source to the second pair of opposed rods are provided whereby a static multipole electric field is created within the filter. At least one source of charged particles is located at one end of the filter for directing said particles into the filter and means for detecting charged particles is located at the end of the filter opposite the charged particle source. An auxiliary electrode is provided located in the central interelectrode space and disposed generally along the central axis of the filter. Mounting means for removably locating the auxiliary electrode in position in the filter is provided and a source of reference voltage is connected by removable means to the auxiliary electrode.

In operation the invention resembles that of the monopole. Charged particles introduced into the filter in an area bounded by pairs of adjacent field electrodes (or rods as they are frequently referred to) and the auxiliary electrode (a quadrant of the filter) follow paths of motion similar to the trajectories or paths of motion followed by particles introduced into a monopole. Such a configuration makes possible the use of multiple sources of charged particles, each directing a stream of particles into one of the four quadrants of the instrument, thereby achieving the capability of continuously and simultaneously observing four difierent species of charged particles in each of the four quadrants of the instrument.

In one embodiment of the invention, the auxiliary electrode is a rod electrode precisely located along the central axis of the filter oriented such that it is parallel to the four field electrodes of the filter. In efi'ect, twoparallel monopole filters are created from a single quadrupole and by proper energization of each of the four field rods and the rod electrode, an analysis and separation capability described above is achieved.

These and other advantages of the invention will be better understood by reference to the following figures in which:

FIG. 1 is a perspective view of the filter of the present invention and associated circuitry having an auxiliary rod electrode disposed along the central axis;

FIG. 2 is a view taken along lines 2-2 of FIG. 1;

FIG. 3 is an alternate embodiment of the filter of this invention utilizing a planar electrode disposed along the central axis of the filter;

FIG. 4 is another alternate embodiment of the filter utilizing a variation of the auxiliary electrode of FIG. 3;

FIG. 5 is a view taken along lines 5-5 of FIG. 3 showing in addition a charged particle source and collector;

FIG. 6 is a perspective view of the preferred embodiment of the present invention;

FIG. 7 is a view taken along lines 77 of FIG. 6; and

FIG. 8 is a side view of the filter shown in FIG. 6 illustrating the manner in which charged particles directed into the filter are detected and recorded.

The monopole mass filter may be described as a one quadrant section of a quadrupole mass filter. The electric fields created in the analyzing region of the monopole are identical to those created in the quadrupole. However, the operation of the two types of filter are considerably different. Basically, the difference resides in the manner in which the stability of ions in the two opposing planes of motion is used. In the quadrupole filter, the transmission of ions for the so-called x-cornponent of motion is eliminated by placing their working point in the x-unstable portion of the stability diagram. In the monopole mass filter, the transmission of ions is not a function of the stability conditions for the x-component of motion. In this type of filter the right angle electrode provided with the single rod or field electrode acts as an obstacle which causes ions to be discharged thereon even when the xcomponent of motion of such a particle falls Within the stable portion of the stability diagram.

The present invention combines the structure and the associated electrical circuitry of a quadrupole and an additional electrode. As shown in FIGS. 1 and 2, the filter comprises a plurality of parallel electrodes 10, normally in the form of elongated cylindrical rods which are symmetrically disposed about a central axis coincident with the Z axis shown in FIG. 1. To opposing pairs of rods are connected sources of DC voltage-s 12 and 14 respectively. A source of alternating voltage 15 is connected by means of a transformer 16 having a center tapped secondary winding such that an alternating voltage of a predetermined polarity is connected to each of opposite pairs of rods of the filter. Disposed along the central or Z axis is a removable electrode 18 which, as shown in FIGS. 1 and 2, is a slender elongated rod extending the length of the filter.

In FIGS. 3 and 4 are shown alternate embodiments of auxiliary electrode 18 of FIGS. 1 and 2. In FIG. 3 the auxiliary electrode 20 is in the form of a planar surface removably mounted by brackets along the central axis of the filter and oriented such that it is parallel to pairs of adjacent rods in the filter. In FIG. 4 the auxiliary electrode 22 is in the form of a planar surface removably mounted by brackets 27 which passes through the diameter of a rod 24, the center of rod generally coinciding with the axis of the filter.

In all of the embodiments shown in FIGS. 1 to 4, the central electrode is connectable to a potential which is the mean of all of the potentials impressed upon the rods. This source of potential will be referred to throughout as a source of reference voltage and normally in the operation of the filter of this invention such a source corresponds to ground potential.

In the embodiment shown in FIGS. 1 and 2, charged particles, normally ions, are directed into the area bounded by adjacent pairs of rods 10 and auxiliary electrode 18. Each of said pairs of rods and auxiliary electrode combine to define four quadrants lying between lines 26, 28, 30 and 32, respectively. Particles to be analyzed are directed into one of the four quadrants where they are subjected to the combined efiect of the alternating and static fields thereby imparting certain trajectories to the particles to eliminate all but those particles having the desired massto-charge ratio; particles having a predetermined mass-tocharge ratio are transmitted by the filter and emerge from the exit end.

In FIG. 5, the view taken along lines 55 of FIG. 3, the side configuration of the auxiliary electrode 20 is shown. In the various embodiments of the invention the central electrode is at least half as long as the overall ength of the filter and is located relative to the rods of the filter such that the exit end 34 of the central electrode 20 coincides with the exit end of the filter. To increase the resolving power of such a filter, the central electrode may be provided with one or more apertures 36, located along the length thereof. Such apertures are preferably located along the central or common axis of the filter and arranged symmetrically with respect to this axis. Charged particles from a source such as that shown at 38 follow a trajectory of motion 40 and emerge from the filter to impinge on a collector 42, causing an electric signal to be transmitted to an amplifying and recording apparatus 43. As the amplitude of the energizing voltages applied to the rods of the filter and/or the frequency of the alternating voltage is varied, the read-out from the filter are indications such as the familiar peaks of a conventional mass spectrum. As is typical of a conventional quadrupole structure, the apparatus of this invention is also energized with symmetrical AC and DC potentials with the DC component being restricted to a value less than 17% of the peak AC component. This restriction on the DC component restrains the ratio of the DC to AC voltage to a value of less than 0.169, the voltage ratio at which the filter theoretically has infinite resolution.

In FIGS. 6, 7 and 8 is illustrated another embodiment of the removable electrode of this invention. As shown in FIG. 6 and 7, the apparatus comprises a plurality of elongated rods 44 arranged symmetrically about a central axis. The rods 44 are elongated cylinders and are disposed in a generally parallel relationship with respect to one another. An auxiliary electrode 46 is disposed within the area bounded by the four rods 44. The auxiliary electrode comprises a pair of planes 48 and 50 which intersect each other at right angles and is disposed such that the line of intersection of planes 48 and 50 coincides with the central axis of the filter. As in the preceding embodiment, the auxiliary electrode 46 is connected by plug and receptacle 47 to a source of reference potential 52, normally ground.

A source of alternating voltage 54 is connected to pairs of rods by means of a transformer 56 whose secondary winding has its center tap 58 connected through a source 60 of fixed potential to the source of reference potential. In this manner, an alternating voltage of opposite polarity is connected to any pair of rods while a unidirectional voltage 60 of a predetermined polarity is connected to all four of the rods of the filter. In FIGS. 6 and 7 the negative side of the unidirectional source is connected to the rods creating negative DC excitation field within the filter. Such an excitation is used when the filter is to be used for analyzing positively charged particles. In the same manner, when the negative side of a DC source is connected to the rods, the filter is used to analyze positively charged particles.

In effect, the structure of the preferred embodiment of the filter results in an apparatus which can be described as four monopoles in parallel. In addition to obtaining four parallel monopoles within a single housing, a serious disadvantage of individual monopoles is eliminated, namely,

the necessity of using a secondary emission electrometer instead of a simple ion collector electrode. In the conventional monopole the single collector electrode is electrically coupled to the field rod which is excited with a high voltage at a high frequency. In contrast, in a filter having an even number of rods the output of the oscillator or AC source is balanced and the potentials of the rods are equal in magnitude and opposite in polarity. Thus the potential induced on the ion collector is zero if electrical and mechanical balance of the filter is maintained.

In FIG. 8 is shown a side view of the filter illustrating two of the four parallel monopoles obtainable with such a structure. Sources 62 of charged particles such as ions are disposed so as to direct these particles along axes 64 through the filter to collectors 66 where the individual signals are amplified in amplifiers 68 and permanently charted at recorders 70.

In addition to other advantages enumerated above, the structure of the present invention is capable of achieving a four-fold increase in sensitivity in comparison to a conventional monopole and by proportioning the voltages applied to the individual rods, simultaneous and continuous response can be obtained for each of four different ionic species in each of the four parallel monopoles. In addition to possession of advantages over a conventional monopole, the structure of this invention possesses the advantage over the conventional quadrupole in that this filter has a less critical dependence upon the constancy of the DC to AC voltage ratio and is characterized by better resolution due to the attenuation of certain types of trajectories.

What is claimed is:

1. A multipole mass filter for selectively detecting charged particles comprising:

four substantially parallel rod electrodes symmetrically disposed about and spaced from a central axis and from each other to define a central interelectrode space located generally along the axis and four remote interelectrode spaces located between each pair of adjacent rod electrodes;

a source of AC voltage;

a source of DC voltage having a positive and a negative side;

a source of reference voltage;

means for applying opposite phases of the AC voltage to a first and second pair of opposed rod electrodes to create an alternating multipole electric field within the filter;

means for applying the positive side of the DC source to the first pair of opposed rod electrodes;

means for applying the negative side of the source of DC voltage to the second pair of opposed rod electrodes whereby a static multipole electric field is created within the filter;

at least one source of charged particles located adjacent one end of the filter for directing said particles into the filter;

means for detecting the presence of charged particles located adjacent the end of the filter opposite the charged particles source;

an auxiliary electrode located in the central interelectrode space and disposed along the central axis of the filter;

mounting means for removably locating the auxiliary electrode in position in the filter; and

means for removably connecting the auxiliary electrode to the source of reference voltage whereby the analyzer is convertible from a quadrupole mass analyzer to a dualpole or monopole mass analyzer.

2. A mass filter according to claim 1 wherein the auxiliary electrode has a configuration such that at least two oppositely located remote interelectrode spaces are free of auxiliary electrode structure.

3; A mass filter according to claim 2 wherein the auxiliary electrode is a planar surface located between adjacent pairs of rods and disposed in a substantially parallel relationship to the plane of a pair of adjacent rods.

4. A mass filter according to claim 3 wherein the planar surface is provided with at least one aperture located along the central axis of the filter.

5. A multipole mass filter according to claim 1, wherein all four remote interelectrode spaces are free of the auxiliary electrode structure.

6. A mass filter according to claim 5 wherein the auxiliary electrode is a cylindrical rod disposed along the central axis.

7. A mass filter according to claim 1 wherein the amplitude of the DC voltage is less than 17% of the peak AC voltage.

8. A multipole mass filter for simultaneously and selectively detecting positively and negatively charged particles comprising:

four elongated substantially parallel rod electrodes symmetrically disposed about a central axis;

a source of AC voltage;

a first source of DC voltage having a positive and a negative side;

a second source of DC voltage having a positive and a negative side;

a source of reference voltage;

means for connecting a phase of the AC voltage to a first pair of opposed rod electrodes;

means for connecting the opposite phase of the AC voltage to a second pair of opposed rod electrodes whereby an alternating multipole electric field is created within the filter;

means for connecting the positive side of the first source of DC voltage to the first pair of opposed rod electrodes;

means for connecting the negative side of the second DC voltage source to the second pair of opposed rod electrodes;

means for connecting the negative side of the first source of DC voltage and the positive side of the second source of DC voltage to the source of reference voltage whereby a static multipole electric field is created within the filter;

at least one source of positively charged particles located adjacent one end of the filter for directing said particles into the, filter;

at least one source of negatively charged particles located adjacent the same end of the filter for directing said particles into the filter; means for detecting the presence of said positively charged particles located adjacent the end of the filter opposite the sources of charged particles;

means for detecting the presence of said negatively charged particles located adjacent the end of the filter opposite the sources of charged particles;

an auxiliary electrode having at least a portion thereof disposed along the central axis of the filter; mounting means for removably locating the auxiliary electrode in position in the filter; and

means for removably connecting the auxiliary electrode to the source of reference voltage whereby the filter is convertible from a quadrupole mass analyzer to a monopole mass analyzer.

9. A multipole mass filter for selectively detecting charged particles comprising:

four substantially parallel rod electrodes symmetrically disposed about a central axis;

a source of AC voltage;

a source of DC voltage having a positive and a negative side;

a source of reference voltage;

means for applying opposite phases of the AC voltage to a first and second pair of opposed rod electrodes to create an alternating multipole electric field within the filter;

means for applyingithe' positive side of the DC source to the first pair opposed rod electrodes; t

means for applying the negative side of the source of DC voltage to the second pair of opposed rod elec-; trodes whereby a static multipole electric fieldis created within the filter;

at least one source of charged particles located adjacent one end of the filter for directing said particles into the filter;

means for detecting the presence of charged particles located adjacent the end of the filter opposite the charged particles source; s

an auxiliary planar electrode passing through the central axis of the filter and disposed in a substantially parallel relationship to the plane of two adjacent rods, the planar electrode having an aperture located at a predetermined point intermediate the ends of the electrode along the central axis;

mounting'means for rer'novably locating the auxiliary electrode in position in the filter; andmeans for connecting the auxiliary electrode to the source of reference voltage whereby the filter is convertible from a quadrupole mass analyzer to a dualpole mass analyzer. 10. A mass filter according to claim 9 wherein the length of the auxiliary'planar electrode is less than the length of the rod electrodes.

References Cited UNITED STATES PATENTS 3,197,633 7 7/1965 Vc-n Zahn 25041.9 3,280,326 10/ 1966 Gunther 25041.9

WILLIAM F. LINDQUIST, Primary Examiner 

